# APU-Physics Project

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Hello, I am an adult returning to school, and I am presently taking phyics. Recently, we were presented with the assignment of writing a report on the "phyics" of a device of our choosing,to be completed by the first of March. I decide to pick the APU ,being an avid flight simmer and hopes of a future in aviation I thought it was the natural choice. I have found a few sites;However,they are very vague,and I am looking for a indepth analysis , and a few close ups to back up my thoughts . So, instead of asking the librarian, I thought I would ask thos who share the same passion as I. If you have any great stories or sites that might have well constructed formula of presentation for such a unit(APU) please share them with me. Thank You. Best Wishes, Robert

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Robert, APU stands for auxiliary power unit, as you probably know, and that term is as diversified as any other general term you could choose. The one common factor is all APU's provide an additional source of "power" to a reliant system. On a Boeing 737 there is a turbine driven APU in the tail cone that uses a small jet turbine engine to turn alternators and compressors to supply 110 Volt 400Hz electrical power as well as compressed air to the aircraft while on the ground. The electrical power is used for lighting, air conditioning, galley appliances, cockpit radios, and other requirements when the plane is not running the main engines. The APU is not able to provide the same amount of power the engines supply but meets most nonflight requirements. The compressed air from the APU is used to start the main engines. The main engines can be started with air from the APU, or electrical power from the APU or a ground unit. On the Space Shuttle Orbiter Vehicles the three APU's are powered by hydrogen which reacts with water to generate electrical current. This power then drives hydraulic and pneudraulic pumps, is conditioned to provide 440 VAC, 400HZ, and 250 VDC to the Orbiter systems. These APU's are different than the 737's though because they stay running during launch and flight and are required for almost all of the shuttles routine operations. On an offshore oil rig you will find APU's that hardly ever run, but must start immediately when needed to provide fire fighting capabilities, evacuation services, emergency radio systems, lighting, and alarm sirens. They are either piston driven auto types engines or jet turbines that will run on either natural gas or gasoline/jet fuel.Now, what type of APU are you looking to write about?Glennformer USAF Aircraft ElectricianPrincipal Electrical Instrument/Controls Designer PriPilot SEL

APU is simply a small turbine engine...Michael J.

No true Michael,As Glenn pointed out an APU can be ANY source of auxillary power. Why I even have one connected to my computer called a UPS.As Glenn also pointed out, the APU can provide power by a mechanical or chemical means. He also mentioned offshore platforms. That just happens to be my area of work. On oil platform complexes you can find giant turbo-generators (a generator powered by a turbine engine) or motor-generators powered by internal combustion type engines (I have seen them be up to 12 cylinder diesels of the type that are used in locomotives.Any device which proveds power either in the absence or failure of main power can be called an APU.

yeah but how does it apply to his question ?. The original question clearly applies to the world of aviation. I never heard of aircraft APU being anything but a turbine.Michael J.

Michael, not quite. Remember the extending propeller driven electricity generators that would deploy in emergencies on WWII era a/c and later models as well? :)EDIT: However, I think all of the answers so far do not define the salient question; what about APU's are you focusing on for you physics paper? There are probably 1,000 different ways you could approach the topic appropriate for a focus on the physics of the beast. What in particular is of interest to you?

I agree Tom. The problem as stated above is poorly defined. Is the main interest in APU itself or is it its power-generation (electricity, pneumatics, hydraulics) qualities ? Also it sounds to me more like an 'engineering' topic - yes, there is a LOT of physics here but so is in a glass of wine ..Michael J.

There is a very good book "Jet aircraft power systems". Amazon has it for \$6.0 (used). You might also find it in your library.Michael J.

I can see it now... ----------------------------[small]Robert ********APU ReportMr. John HancockFebruary 6, 2004[/small]

Auxillary Power Units

APU is simply a small turbine engine...

cloud>----------------------------------------MLA style report was what I was aiming at... a seven word report! :-lol That'd be hilarious :D :-lolP.S. - gotta love the sound of the APU!

>EDIT: However, I think all of the answers so far do not define>the salient question; what about APU's are you focusing on for>you physics paper? There are probably 1,000 different ways you>could approach the topic appropriate for a focus on the>physics of the beast. What in particular is of interest to>you?My point exactly, Tom. I wanted to let Robert know that the general term, "APU", was a very broadly applied term and he should consider narrowing his subject matter a bit to produce a narrative specific, or to diversify his research and do a general, all encompassing type "introduction to" essay.For a college student this kind of pre-project awareness can be the difference in a successful grade and an embarassing mention in front of the class. Robert asked, I related what I knew.Thanks for your support.Glenn

>yeah but how does it apply to his question ?. The original>question clearly applies to the world of aviation. I never>heard of aircraft APU being anything but a turbine.The Canadiar CL-215 uses a non-turbine APU. It uses a little 2 cylinder gas engine that is pull-started, just like a lawnmower!

A number of the larger allied aircraft in WWII had APUs. A good number of B-17Fs and B-17Gs had small gasoluine engines in the aft fuselage that drove generators. Not typical, though. B-24s, and B-29s had them also, and a few of the large British types had them as well. The Spruce Goose had a pretty big APU(a gasoline or diesel, i'm not sure) used to provide power and other services while the right engines weren't running. The USS Akron and USS Macon(airships) had several auxilary enignes per ship dedicated to generating power for lights, radios, etc. APUs are not intended top provide thrust, but i've heard that early 747 pilots would very rarely leave the APU on during takeoff to provide 1,000 lbs more thrust during high weight takeoffs.

>>EDIT: However, I think all of the answers so far do not>define>>the salient question; what about APU's are you focusing on>for>>you physics paper? There are probably 1,000 different ways>you>>could approach the topic appropriate for a focus on the>>physics of the beast. What in particular is of interest to>>you?>>My point exactly, Tom. I wanted to let Robert know that the>general term, "APU", was a very broadly applied term and he>should consider narrowing his subject matter a bit to produce>a narrative specific, or to diversify his research and do a>general, all encompassing type "introduction to" essay.>>For a college student this kind of pre-project awareness can>be the difference in a successful grade and an embarassing>mention in front of the class. Robert asked, I related what I>knew.>>Thanks for your support.>>Glenn > Hello, Glenn and Tom, Thank you for your kind replies, I will follow your instructions and narrow my research down focusing on one specific area of the APU.I believe I will concentrate on the APU 's "Turbine operations" in commercial aircrafts such as: "..On a Boeing 737 there is a turbine driven APU in the tail cone that uses a small jet turbine engine to turn alternators and compressors to supply 110 Volt 400Hz electrical power as well as compressed air to the aircraft while on the ground.." Glenn There is much research to be done! Michael J.,Thank you for the reference to the "Jet aircraft power systems". Best Wishes , Robert

"On a Boeing 737 there is a turbine driven APU in the tail cone that uses a small jet turbine engine to turn alternators and compressors to supply 110 Volt 400Hz electrical power as well as compressed air to the aircraft while on the ground."Glenn....Isn't there only one generator on a 737 and isn't it supposed to supply a nominal 115Vac?Also, what do you mean by compressors? There is only one "load" compressor (which is basically the front part of the spinning engine rotor)"The main engines can be started with air from the APU, or electrical power from the APU or a ground unit."I think you'll find that the main engines on a 737 cannot be started with electrical power from the APU ;-) (at least not on commercial 737's).Rgds.Ian.

Hello, I justed wanted to thank thos who have given me information, and ideas about the APU,especially Glenn and Tom. I have just received my mark for my term paper on the APU and got an "A" 90% YAHOOOOOOOOOOOO!:D Best Wishes , Robert

The STS orbiter APUs are only operated during ascent, rentry and landing phases of the flight to operate flight controls, landing gear extension, and gimbaling of rocket engines. I'm not sure if a seperate system is used for the gimbals on the outlets at the base of the solid boosters. Since they are operated at such high altitudes, the orbiter must provide seperate fuel and oxidizer tanks to run the redundant APU turbines. The APUs and supply tanks are located in the space betwen the cargo bay and the main engines. Main engine propelents are pumped by seperate turbines that operate from materials diverted from the engine propelant supply, which is no longer available after the main engines are shut down.The APUs are shut down during orbital operations and restarted shortly before the deorbit "Orbiting Maneuvering System" burn. In orbit, electrical power is generated by a chemical process that produces water and electricity from hydrogen and oxygen in a unit refered to as a "Fuel Cell". The process is particularly convenient, since the water byproduct can be used for food preperation (rehyrating dried products), water for crew and eating utensil cleaning and the crew's drinking water supply. Excess heat is radiated to space by equipment located on the interior surface of the cargo bay doors, which must be opened soon after reaching orbit. If the cargo bay doors cannot be promptly opened, the flight has to be aborted and the orbiter has to promptly return to Earth. A malfunctioning cryogenic oxygen supply tank for a similar fuel cell system is what exploded in the Apollo 13 service module.

Tom, What you are describing is a ram air turbine ( RAT ) in modern parlance and no way does that constitute an APU. You must have airspeed for a rat to operate! A RAT is typically used as an emergency device. First generation jet aircraft ( and some later piston engined aircraft) required ground based generators driven by large GM diesels to supply 115/200 v ac power for maintenance/starting operations. Also a mobile gas turbine was required to supply air for the same reasons. This dependence on these units severly limited the operational use of the aircraft as it required airlines to have ground crews and equipment at all airports. A very expensive requirement. The APU (auxillary power unit) is used to overcome the previous limitations. Typically a modern APU consists of a small gas turbine engine ( electric starter) driving an accessory drive gearbox that has generators (AC) and a hydraulic pump, and also uses bleed air to supply electrical/hydraulic/pneumatic power to the aircraft systems for maintenance/starting.From memory the military were the originators of the APU so that they could scramble aircraft without need for ground support equipment. Of course civil aviation soon saw the dollars and cents advantage. Whilst an APU MAY (under some circumstances) be used as an emergency supply this is not it's main reason for being. Roger

Way to go Robert. We got an "A" huh?:>)Good work, glad I could help in some small way.Glenn

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